Knife sharpener



FIG. 1 is a top, front and left side perspective view of a knife sharpener showing our new design;

FIG. 2 is an enlarged left side elevational view thereof, the right side being a mirror image;

FIG. 3 is an enlarged front elevational view thereof;

FIG. 4 is an enlarged rear elevational view thereof;

FIG. 5 is an enlarged top plan view thereof; and,

FIG. 6 is an enlarged bottom plan view thereof. 

The ornamental design for a knife sharpener, as shown and described. 